(A) Field of the Invention
This invention relates to analytical methods for detecting and quantifying carbon isotopes and more particularly relates to the quantification of carbon isotopes by mass spectrometry.
(B) History of the Prior Art
The most common method for detecting and quantifying carbon 14 is scintillation counting of the radioactive decomposition of the carbon 14 isotope. The non-radioactive and more common .sup.12 C and .sup.13 C isotopes cannot be determined by this method since they are non-radioactive.
Amounts of carbon 14 of less than 1.times.10.sup.-11 gram cannot be quantitatively determined by scintillation counting in less than five minutes with a statistical precision of 5% or better and quantities of less than 1.times.10.sup.-12 (one trillionth) gram of carbon 14 cannot be quantitatively determined by scintillation counting in less than one hour with a statistical precision of 5% or better. When a statistical precision of 1% or better is required, it would take over two hours to quantitatively determine less than 1.times.10.sup.-11 gram of carbon 14 and over a day to quantitatively determine less than 1.times.10.sup.-12 gram of carbon 14. Determination of quantities below 1.times.10.sup.-12 gram of carbon 14 becomes completely impractical by this method over any period of time due to the interference of background radiation.
The detection and quantification of carbon 14 in an amount even as low as 1.times.10.sup.-12 gram are quantities which are undesirably high to permit the use of carbon 14 as an in vivo biological radioactive tracer in humans. In order to obtain a sample from the person upon which a tracer study is being made, which contains an analyzable quantity of carbon 14, the person must be exposed to a much larger quantity of carbon 14. All of the carbon 14 injected into the person would almost certainly not become concentrated in the sample taken from the person and, in addition, since the sample being considered must be adjusted for the biological half life of the compound, a substantial excess of carbon 14 would initially be required to compensate for loss due to biological excretion or biochemical transformation.
These considerations could therefore easily require an exposure to carbon 14 of 100,000 to 1,000,000 times the carbon 14 contained in the sample obtained for analysis. The quantity required to be injected becomes even greater when shorter times for analysis (e.g., less than 5 minutes) are required due to short residence time in automated analytical equipment. Furthermore, when better analytical precision is desired (e.g., a statistical precision better than 5%), even higher quantities of carbon 14 must be injected.
In addition, scintillation counting is an unsatisfactory method of dating materials by carbon 14 decay due to interference of background radiation and due to the inability of this method to determine carbon 14 to carbon 12 ratios at small sample size within a reasonable period of time, even if the effects of background radiation could be sufficiently reduced, e.g., by conducting the method in a salt mine.
Prior art carbon dating was originally accomplished by gas phase counting, (a method less sensitive than scintillation counting) and was then subsequently accomplished by scintillation counting which is a method which is not as sensitive as desired as previously discussed.
Carbon 14 dating has been accurately accomplished by the use of particle accelerators to obtain highly positively charged carbon atoms which were then separated by mass spectrometry and then directly or indirectly counted. This method requires extremely costly equipment and requires relatively large sample sizes.
An attempt has been made to determine carbon 14 by converting carbon to CO.sub.2, mixing the CO.sub.2 with nitrogen, converting the mixture to CN.sup.- ions and determining the quantity of .sup.14 C.sup.15 N.sup.- by mass spectrometry (U.S. Pat. No. 3,885,155). Unfortunately, this method is insufficiently sensitive for use in carbon 14 biological tracer studies and in carbon 14 dating tests due to interference of ionic species of essentially the same charge to mass ratio as .sup.14 C.sup.15 N.sup.-. Particularly troublesome interfereing ions are .sup.29 Si.sup.- and .sup.28 S.sub.i H.sup.-. The silicon is difficult, if not impossible, to sufficiently exclude since it forms a part of the material of the construction of the instrument, i.e., as an impurity in the metallic materials or as a component in glass.